The present invention relates to thermometry and more particularly to thermometers that are more accurate and faster acting. Thermometers may be classified as invasive, where the thermometer is placed in a body cavity such as the rectum, under arm or mouth or non-invasive where the thermometer does not enter the body cavity but at most, contact's the subject's skin. Non-invasive thermometers are growing in popularity both because of their ease of use and gentleness to the subject. A common type of non-invasive thermometer includes a probe with a heat conducting membrane designed to be placed against the skin of a subject's temple, behind the ear or other body surface. An early version of such a thermometer utilized a probe to obtain temperature readings at the measuring site and an algorithm to utilize parameters derived form the measured temperature to correct a fixed bias to a reasonable proximation and clinically accepted value of the subject's core temperature, that is the temperature of blood flowing in the pulmonary artery. An improvement on this thermometer is disclosed in U.S. Pat. No. 7,597,668 to Yarden wherein a deep tissue temperature, that is, the local temperature below the skin surface at the measuring site that is the source of heat to the probe is calculated utilizing parameters derived from the measured temperatures and an algorithm is utilized to correct the calculated deep tissue temperature to core. Non-invasive temperature measurement of a deep tissue is challenging. One can measure it with commonly acceptable accuracy using a well insulated contact temperature sensor attached to the external surface above the deep tissue. When the temperature sensor is reaching to its equilibrium, the temperature value at steady state is approaching to the deep tissue temperature value and is a good representation of it. However, in some thermometers, the steady state value of the temperature sensor can be calculated within a shorter time than required to reach equilibrium. This calculation is called prediction, i.e. the thermometer is predicting the steady state value of the sensor before it reaches to the steady state and might be determined using prediction algorithms such as described in U.S. Pat. No. 4,866,621 and U.S. Pat. No. 4,592,000.
Once the local temperature (which is the steady state value of a surface temperature sensor) is determined, further algorithms are used to correct the local temperature to core.
Other non-invasive thermometers utilize IR sensors to determine the surface temperature at a measuring site along with an algorithm to convert parameters derived from the measured surface temperatures to core temperature. That is, the local or steady state value of the skin temperature is measured and then corrected to reflect the core body temperature. Such a thermometer, for example, is disclosed in U.S. Pat. No. 6,292,685 to Pompei.
An assumption in the algorithms of exiting non-invasive thermometers for converting parameters derived from temperature measurements at the measuring site to core temperature is that physiological factors other than the subject's temperature are the same or closely similar for all subjects, regardless of age, skin tone, weight, etc. That is, the assumption is that the relationship between the steady state temperature at the measuring site and a subject's core temperature is only thermal. However, it has been found that other physiological characteristics of the subject's anatomy come into play, such as the thermal conductivity, thermal impedance and blood perfusion of the subject's skin and tissue at the measuring site.
In view of the above it is a principal object of the present invention to provide an improved thermometer capable of more accurately and/or more rapidly determine the steady state temperature at a measuring site.
A further object is to provide such a thermometer that is able to rapidly calculate core temperature from the temperature and other non-temperature physiological parameters obtained preferably but not necessarily at the measuring site.